This invention relates to speed control apparatus of a polyphase induction motor, while maintaining the slip frequency at a constant value.
In order to control the speed of an induction motor while maintaining the slip frequency at a constant value, it has been the practice to use a tachometer generator as a speed detector for producing a speed signal and a primary frequency signal of the motor. However, since a tachometer generator cannot detect the angular position of the rotor, it is necessary to add some means for detecting the angular position of the motor to provide an efficient speed control.